Unison rings are provided on the axial compressor sections of modern gas turbine engines to allow adjustment of the compressor stator vane angle during operation of the engine. In simple terms, each stator vane in an individual compressor stage is provided with a mounting pivot disposed in the compressor housing and oriented so as to permit rotation of the stator vane about its longitudinal axis. Simultaneous movement of the vanes in an individual stage is accomplished through the use of a unison ring, disposed circumferentially about the exterior of the compressor housing and linked to each stator vane by individual vane lever arms which rotate each vane about its corresponding pivot in response to the tangential displacement or rotation of the unison ring.
Typical gas turbine engines utilize a plurality of compressor stages, each stage comprising a set of stator vanes for receiving and redirecting the air or gas issuing from the rotating blades of the preceding stage. For gas turbine engines operating at varying speeds and inlet conditions, such as those used in the aircraft industry, it is particularly beneficial to alter the angle of attack of the individual stage stator vanes depending upon the current engine operating speed and conditions.
Typical gas turbine engines thus include two or more stages of adjustable stator vanes, each having a corresponding unison ring. The unison rings are usually adjusted by a single actuator assembly, the actuator assembly displacing the individual unison rings tangentially in response to engine speed, power requirement, or other operating parameters in order to achieve dependable and efficient operation. As typical unison ring operation schedules call for simultaneous movement of the individual unison rings in response to the selected parameter or parameters, it is therefore common to utilize a single drive component to initiate the displacement of the individual unison rings. This drive component, such as a linear hydraulic piston actuator, is mounted to the exterior of the compressor housing and acts against the drive arm of a bellcrank which is also mounted to the compressor housing and rotatable about an axis parallel to the longitudinal axis of the compressor. A plurality of pushrods connect the individual unison rings to corresponding crank arms on the rotatable bellcrank, thus moving the rings in response to the rotation of the bellcrank under the influence of the linear drive component. A typical actuation system according to the prior art is disclosed in U.S. Pat. No. 4,403,912 "Integrated Multiplane Actuator System for Compressor Variable Vanes and Air Bleed Valve".
As would be expected with actuator systems supported about the periphery of a compressor housing or the like, the transfer of loads to the housing is of particular concern, with care being taken to avoid the imposition of excessive radial forces which may deform the lightweight housing. As would be readily appreciated by those familiar with axial gas compressors, the clearance between the rotating compressor blades and the generally cylindrical compressor housing must be minimized in order to achieve acceptable compressor operating efficiency. Such clearance may be reduced or otherwise compromised by local deformation of the compressor housing either inwardly or outwardly as the result of local radial or bending forces imparted to the compressor housing by the unison ring actuator.
In the past, the loading of the compressor housing has been addressed primarily through the use of local bracing and other well known methods of distribution the imposed stress. This approach, while successful and still currently in use, has added components, complexity, and weight to the final assembly.
It has further been found that such engines profit by the non-proportional movement of the individual stator vanes. The achievement of such non-proportional actuation between the individual stator stages has required engine designers to provide an increased radial displacement between the compressor housing and the bellcrank pivot, further increasing the bending stress on the bellcrank mountings and likewise on the compressor housing. The concurrent increase in size of the drive component has likewise increased its radial displacement relative to the compressor housing thus multiplying the loads imposed on the drive component mounting brackets.
In addition, deflections of the compressor case and bellcrank mounting affect the accuracy of the actuation system, a distinct disadvantage when even a few degrees of vane angle error may significantly reduce compressor efficiency. Such accuracy may also be influenced by the differential thermal expansion of the various components as the engine is heated and cooled throughout the operating cycle.
What is required is an actuator for imparting non-proportional tangential displacement to a plurality of compressor unison rings which does not impose undesirable radial forces or local bending moments upon the compressor housing, and which minimizes positional inaccuracy of the individual stator vane stages due to component deflection under load or differential thermal expansion.